This invention relates to a regeneration type fuel pump for use in a vehicle such as a motor vehicle and, more particularly, to a fuel pump for supplying fuel to an injector at a high pressure.
In a conventional fuel pump, such as the one disclosed in Japanese Patent Unexamined Publication No. 60-173389, an impeller is accommodated in a pump housing and is fixed to an armature. As shown in FIG. 12, the impeller 10 has a plurality of grooves 11 formed in its outer circumferential portion. As the impeller 10 rotates, fuel is pumped out and supplied to an external injector by being moved from each groove 11 to the adjacent groove 11 in one direction while being pressurized.
In fuel pumps ordinarily used for vehicles, an air intake pipe pressure is introduced as a back pressure of a pressure regulating valve to constantly maintain the fuel pressure at a level higher than the intake pipe pressure by a set pressure (2.55 kg/cm.sup.2), thereby definitely determining the injection rate with the injector energization time.
Such fuel pumps are driven by a battery mounted on the vehicle at a voltage ranging from 12 to 14 V, and the flow rate of the fuel pump is set within a range (50 to 200 l/h) predetermined correspondingly.
However, it has been found that at the time of starting, in particular, a cold start, the voltage of the vehicle battery is so low, about 8.5 V, that the flow rate is considerably reduced and that, in the worst case, the flow rate is lower than 20 l/h which is the lowest flow rate necessary for starting the engine. This lowest necessary flow rate is required to expel fuel vapor accumulated in the fuel piping during stoppage. If the flow rate is not maintained at or above this level, the engine cannot be suitably started.
To cope with this problem, according to the above-mentioned prior art, the number of revolutions of the impeller may be increased in order to increase the delivery flow rate. To increase the number of impeller revolutions, however, it is necessary to increase the motor driving voltage. The motor load is thereby considerably increased.